This invention relates to an ion beam source, and more particularly to a point ion beam source.
The performance of ion beam application apparatuses or systems such as an ion beam micro-analyzer, an ion beam lithography system for drawing a semiconductor circuit, etc. greatly depends upon the capacity of ion beam source. The ion beam source used in these apparatuses or systems is a point ion beam source that emits a narrow beam. The necessary conditions for the point ion beam source are that a stable ion beam having a high brightness be emitted for a prolonged duration.
Ion species include ions of metal such as gallium (Ga), indium (In), bismuth (Bi), gold (Au), lead (Pb), iron (Fe), cobalt (Co), nickel (Ni), etc., ions of metalloid and semiconductor elements such as boron (B), phosphorus (P), arsenic (As), antimony (Sb), silicon (Si), etc.; ions of gases such as hydrogen (H.sub.2), helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), nitrogen (N.sub.2), oxygen (O.sub.2), water (H.sub.2 O), methane (CH.sub.4), etc.
To produce ions of the said species, it is necessary to apply a high positive electric field to a needle-like tip, and supply a liquid metal of Ga, In, or the like, or a gas of H.sub.2, He, Ar, O.sub.2, or the like to the tip to ionize it under the action of electric field. However, among the metalloid and semiconductor elements, B and Si have a high melting point and P, As and Sb have a very high vapor pressure in a liquid state. Thus, to obtain ions of these elements, a liquid alloy is usually used. For example, an alloy consisting of 45% by weight of B and 55% by weight of Ni has a melting point of about 1,000.degree. C. and also has a low vapor pressure. When the alloy is ionized, B ions and Ni ions can be obtained, and only B ions can be isolated through a magnetic field.
To obtain B ions, alloys of B--Pt, B--Ni--Pt, etc. are available in addition to the alloy of B--Ni. To obtain ions of Si, P, As, and Sb, alloys of Si--Au, P--Ni, As--Ni, As--Pt, and Sb--Au are available. The alloy systems are, however, not limited to the above-mentioned ones, and alloys of any combination are available in principle, so long as the combination has a lower melting point and a low vapor pressure, when alloyed.
In the case of a liquid metal including a liquid alloy, such a procedure is used that a liquid metal supply source is provided at the bottom of a needle-like tip and the liquid metal is diffused on the tip surface along the axis of tip to supply the liquid metal to the end of the tip from which ions are emitted.
In the case of a gas, such a procedure is usually used that a gas is introduced into a chamber encasing a needle-like tip to supply the gas to the tip. To obtain a gas ion beam having a high brightness, such a procedure is used that the needle-like tip is cooled to less than the liquefaction temperature of the gas to condense it on the tip surface and diffuse the tip surface on the same principle as in the case of the liquid metal, thereby supplying the condensed gas molecules to the end of the tip followed by ionization.
Requirements for the needle-like tip that ionizes a liquid metal or a gas in the foregoing manner are that (1) it withstands a high electric field, (2) it is not eroded by a liquid metal or gas molecules to be ionized, (3) it is wetted by a liquid metal or gas molecules to some extent, (4) it undergoes efficient ionization at the tip, and (5) it is electro-conductive. In addition, practical requirements are that it can be readily processed into a needle-like tip, and also can be readily handled.
The needle-like tip so far used is restricted to tips of refractory metals such as tungsten (W) and molybdenum (Mo). Particularly, needle-like tip of W has been used as a practical point ion beam source, because W has the highest melting point among the metals, the highest strength under the working conditions of electric field, and a good processability.
However, the ionization of gases, particularly, H.sub.2, He, Ar, Kr, Xe, O.sub.2, etc. suffers from such problems that the strength of W against the electric field is not high enough, and the shape of needle-like tip is deformed as a result of reaction of W with an active gas such as H.sub.2, O.sub.2, etc. To improve the life of ion beam source and the stability of emitted ion beam, it is necessary to use a needle-like tip having a higher strength against the electric field and a less reactivity particularly with an active gas than W.